This invention relates generally to digital speech communication systems and particularly to such a system having feedback residue compression in its predictive feedback loop.
Bandwidth compression for speech signals has generally been accomplished in two different manners. Thus, the compression may be accomplished by the time domain techniques which operate at relatively high bit rates of between 16-40 kilobits per second. Among these time domain techniques are delta modulators where the difference between the estimate of the predictive feedback and the actual input is small. The other bandwidth compression techniques are spectral domain techniques such as vocoders. These systems operate at very low bit rates between 2.4 and 4.8 kilobits per second.
The spectral domain systems are susceptible to errors induced by background noise. This is a result of their restrictive manner of compressing the signal input. Due to their low bandwidth they do not permit to preserve the fidelity of speech.
It is therefore desirable to provide a speech processing system which compresses both the speech and the noise and possibly giving preference to the speech. For this reason time domain techniques appear to have certain advantages.
Among the systems are continuously variable slope delta modulation techniques. This simply means that the slope or the size of the increment can be changed or varied.
Other time domain systems which are characterized by relatively low data rates are adaptive predictive coding systems. In particular the adaptive predictive coding system is characterized by higher intelligibility of the speech and speech quality than can be achieved at lower data rates than can be utilized with delta modulation. However, one of the problems with the adaptive predictive coding system is the complexity of the required hardware.
It is accordingly an object of the present invention to provide a digital speech compression system of the type having a predictive feedback loop and which is characterized by greater simplicity.
A further object of the present invention is to provide such a speech compression system where the compression is achieved by limiting the number of sequences of quantizer levels that may be fed back to the loop.
Another object of the present invention is to provide a predictive speech compression system where the residue stream is compressed within the quantizer loop.